A practical synthesis of 2-deoxy-2-fluoro-D-arabinofuranose derivatives.

A seven-step synthesis of 1,3-di-O-acetyl-5-O-benzoyl-2-deoxy-2-fluoro-D-arabinofuranose, a versatile intermediate in the synthesis of chemotherapeutically important nucleosides, was achieved from 1,2:5,6-di-O-isopropylidene-3-O-tosyl-alpha-D-allofuranose. The crucial steps were the fluorination by use of potassium fluoride in acetamide and the conversion of 6-O-benzoyl-3-deoxy-3-fluoro-D-glucofuranose into 5-O-benzoyl-2-deoxy-2-fluoro-3-O-formyl-D-arabinofuranose by periodate oxidation. Also described is the synthesis of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)cytosine. This procedure affords good overall yields of products without formation of undesirable, isomeric intermediates and is suitable for large-scale preparations.     

Acyloxy neighboring-group participation in the acid-catalyzed cleavage of methyl 2,3-anhydro-beta-D-ribofuranoside.

The reaction of methyl 2,3-anhydro-beta-D-ribofuranoside with hydrogen bromide in an acetic acid-acetic anhydride solution leads to the formation of methyl 2,3-di-O-acetyl-5-bromo-5-deoxy-alpha,beta-D-xylofuranoside. Similar treatment of methyl 2,3-anhydro-5-O-benzoyl-beta-D-robofuranoside provided methyl 2-O-acetyl-3-O-benzoyl-5-bromo-5-deoxy-alpha,beta-D-xylofuranosides. The position of halogen substitution was ascertained by hydrogenolysis to the resultant 5-deoxy sugars, which were characterized by their n.m.r. spectra. Confirmation of the structural assignment for methyl 2-O-acetyl-3-O-benzoyl-5-deoxy-alpha,beta-D-xylofuranoside was obtained by synthesis from 1,2-O-isopropylidene-alpha-D-xylofuranose. The formation of the 5-bromo derivatives under the reported conditions probably occurred through the intermediacy of the 3,5-acyloxonium ions. Similar conversions were observed when the starting compound was treated with hydrogen chloride, acetyl bromide, or acetyl chloride in acetic acid-acetic anhydride solutions.     

A practical route to partially protected pyrrolidines as precursors for the stereoselective synthesis of alexines

Either 3-O-benzoyl- (2a) or 3-O-benzyl-1,2-O-isopropylidene-beta-D-fructopyranose (2b) were regioselectively O-benzylated at C-4 to give 4a and 4b, respectively, which were transformed into 5-azido-3-O-benzoyl-4-O-benzyl- (6a) and 5-azido-3,4-di-O-benzyl-5-deoxy-1,2-O-isopropylidene-alpha-L-sorbopyranose (6b) by nucleophilic displacement of the corresponding 5-O-mesyl derivatives 5a and 5b by sodium azide in DMF, respectively. Compound 6b was also prepared from 4b in one step by the Mitsunobu methodology. Deacetonation of 6a and 6b gave the partially protected free azidouloses 8a and 8b, respectively, that were protected as their 1-O-TBDPS derivatives 9a and 9b. Hydrogenation of 9b over Raney nickel gave stereoselectively (2R,3R,4R,5S)-3,4-dibenzyloxy-2'-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (12) which was identified by transformation into the well known (2R,3R,4R,5S)-3,4-dihydroxy-2,5-bis(hydroxymethyl)pyrrolidine (1, DGDP).     

Synthesis of 3-C-(methyl beta-D-xylofuranosid-3-yl)-5-phenyl-1,2,4-oxadiazole.

Nucleophilic addition of KCN to 5-O-benzoyl-1,2-O-isopropylidene-alpha-D-erythro-pentofuranos-3-++ +ulose gave the xylo cyanohydrin stereoselectively. Several xylos-3-yl-1,2,4-oxadiazole derivatives were synthesized from this cyanohydrin and were converted into 3-C-(methyl beta-D-xylofuranosid-3-yl)-5-phenyl-1,2,4-oxadiazole.     

Synthesis of 1,3-Dl-O-Acetyl-5-O-Benzoyl-2-O -(O-Carboran-1-Ylmethyl)- D-Ribofuranose. A General Precursor for the Preparation of Carborane-Containing Nucleosides for Boron Neutron Capture Therapy

Abstract

An eight-step synthesis of 1,3-di-O-acetyl-5-O-benzoyl-2-O-(o-carboran-1-ylmethyl)-D-ribofuranose 9 was carried out from 1,2:5,6-O-isopropylidene-α-D-allofuanose 1. Condensation of 9 with trimethylsilyl protected uracil in the presence of trimethylsilyl trifluoro-methanesulfonate, and subsequent deblocking of the resulting 1-[3-O-acetyl-5-O-benzoyl-2-O-(o-carboran-1-ylmethyl)-D-ribofuranosyl]uracil 10 (>95& β-configuration) by alkaline hydrolysis, yielded 1-[2-O-(o-carboran-1-ylmethyl)-β-D-ribofuranosyl]uracil 11.     

Studies on the stereoselective synthesis of a protected α-D-Gal-(1→2)-D-Glc fragment

A novel 1,2-cis stereoselective synthesis of protected α-D-Gal-(1→2)-D-Glc fragments was developed. Methyl 2-O-acetyl-3-O-allyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3-O-benzoyl-4,6-O-benzylidene-α-D-glucopyranoside (13), methyl 2-O-acetyl-3-O-allyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-α-D-glucopyranoside (15), methyl 2-O-acetyl-3-O-allyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3-O-benzoyl-4,6-O-benzylidene-β-D-glucopyranoside (17), and methyl 2-O-acetyl-3-O-allyl-4,6-O-benzylidene-α-D-galactopyranosyl-(1→2)-3,4,6-tri-O-benzoyl-β-D-glucopyranoside (19) were favorably obtained by coupling a new donor, isopropyl 2-O-acetyl-3-O-allyl-4,6-O-benzylidene-1-thio-β-D-galactopyranoside (2), with acceptors, methyl 3-O-benzoyl-4,6-O-benzylidene-α-D-glucopyranoside (4), methyl 3,4,6-tri-O-benzoyl-α-D-glucopyranoside (5), methyl 3-O-benzoyl-4,6-O-benzylidene-β-D-glucopyranoside (8), and methyl 3,4,6-tri-O-benzoyl-β-D-glucopyranoside (12), respectively. By virtue of the concerted 1,2-cis α-directing action induced by the 3-O-allyl and 4,6-O-benzylidene groups in donor 2 with a C-2 acetyl group capable of neighboring-group participation, the couplings were achieved with a high degree of α selectivity. In particular, higher α/β stereoselective galactosylation (5.0:1.0) was noted in the case of the coupling of donor 2 with acceptor 12 having a β-CH(3) at C-1 and benzoyl groups at C-4 and C-6.     

3'-deoxyribofuranose derivatives of 1-deaza and 3-deaza-adenosine and their activity as adenosine deaminase inhibitors

2,6-Dichloro-1-deazapurine and 2,6-dichloro-3-deazapurine were coupled with 1,2-O-diacetyl-5-O-benzoyl-3-deoxy-beta-D-ribofuranose. Deprotection of the obtained compounds and reaction with liquid ammonia gave the desired 2-chloroadenine nucleosides, which were dechlorinated to afford the corresponding 1-deaza and 3-deazaadenosine derivatives. Biological studies performed on ADA from calf intestine showed that these new nucleosides are inhibitors of the enzyme.     

A concise synthesis of 3-fluoro-5-thio-xylo- and glucopyranoses, useful precursors towards their corresponding pyranonucleoside derivatives

The chemical synthesis of 1,2,4-tri-O-acetyl-3-deoxy-3-fluoro-5-thio-D-xylopyranose, 1,2,4,6-tetra-O-acetyl-3-deoxy-3-fluoro-5-thio-alpha-D-glucopyranose and their corresponding nucleosides of thymine is described. Treatment of 3-fluoro-5-S-acetyl-5-thio-D-xylofuranose, obtained by hydrolysis of the isopropylidene group of 3-fluoro-1,2-O-isopropylidene-5-S-acetyl-5-thio-D-xylofuranose, with methanolic ammonia and direct acetylation, led to triacetylated 3-deoxy-3-fluoro-5-thio-D-xylopyranose. Condensation of acetylated 3-fluoro-5-thio-D-xylopyranose with silylated thymine afforded the corresponding nucleoside. Selective benzoylation and direct methanesulfonylation of 3-fluoro-1,2-O-isopropylidene-alpha-D-glucofuranose gave the 6-O-benzoyl-5-O-methylsulfonyl derivative, which on treatment with sodium methoxide afforded the 5,6-anhydro derivative. Treatment of the latter with thiourea, followed by acetolysis, gave the 3-fluoro-5-S-acetyl-6-O-acetyl-1,2-O-isopropylidene-5-thio-alpha-D-glucofuranose. 3-fluoro-5-S-acetyl-6-O-acetyl-5-thio-D-glucofuranose, obtained after hydrolysis of 5-thiofuranose isopropylidene, was treated with ammonia in methanol and directly acetylated, giving tetraacetylated 3-deoxy-3-fluoro-5-thio-alpha-D-glucopyranose. Condensation of the latter with silylated thymine afforded the desired 3-deoxy-3-fluoro-5-thio-beta-D-glucopyranonucleoside analogue.     

A precursor to the beta-pyranosides of 3-amino-3,6-dideoxy-D-mannose (mycosamine).

SN2-type reaction of 3-O-(1-imidazyl)sulfonyl-1,2:5,6-di-O-isopropylidene-alpha-D-gluco furanose with benzoate gave the 3-O-benzoyl-alpha-D-allo derivative 2, which was hydrolysed to give the 5,6-diol 3. Compound 3 was converted into the 6-deoxy-6-iodo derivative 4 which was reduced with tributylstannane, and then position 5 was protected by benzyloxymethylation, to give 3-O-benzoyl-5-O-benzyloxymethyl-6-deoxy-1,2-O-isopropylidene-alpha -D- allofuranose (6). Debenzoylation of 6 gave 7, (1-imidazyl)sulfonylation gave 8, and azide displacement gave 3-azido-5-O-benzyloxymethyl-3,6-dideoxy- 1,2-O-isopropylidene-alpha-D-glucofuranose (9, 85%). Acetolysis of 9 gave 1,2,4-tri-O-acetyl-3-azido-3,6-dideoxy-alpha,beta-D-glucopyranose (10 and 11). Selective hydrolysis of AcO-1 in the mixture of 10 and 11 with hydrazine acetate (----12), followed by conversion into the pyranosyl chloride 13, treatment with N,N-dimethylformamide dimethyl acetal in the presence of tetrabutylammonium bromide, and benzylation gave 3-azido-4-O-benzyl-3,6-dideoxy-1,2-O-(1-methoxyethylidene)-alpha-D -glucopyranose (15). Treatment of 15 with dry acetic acid gave 1,2-di-O-acetyl-3-azido-4-O-benzyl-3,6-dideoxy-beta-D-glucopyranose (16, 86% yield) that was an excellent glycosyl donor in the presence of trimethylsilyl triflate, allowing the synthesis of cyclohexyl 2-O-acetyl-3-azido-4-O-benzyl-3,6-dideoxy-beta-D-glucopyranoside (17, 90%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis of the capsular polysaccharide of Streptococcus pneumoniae type 14. 2. Synthesis of methyl-6-O-acetyl-4-O-(2,3,46-tetra-O-benzoyl -beta-D-galactosylpyranosyl)-2-deoxy-2-phthalimido-beta-D-gluco- pyranoside--a lactosamine precursor in the monomer synthesis for polycondensation

Glycosylation of methyl 6-O-acetyl-3-O-benzoyl-2-deoxy-phthalimido-beta-D-glucopyranoside and its 4-trityl ether by benzobromogalactose, 1-O-acetyl-2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranose, 1,2-[(alpha-p-tolylthio)benzylidene]- and 1,2-O-[(alpha-cyano)benzylidene]-3,4,6-tri-O-benzoyl-alpha-D- galactopyranoses proceeds non-stereospecifically. The best yield of beta-linked disaccharide was obtained upon glycosylation by benzobromogalactose in the presence of silver triflate and tetramethylurea in nitromethane.     

Synthesis of (Z)-3,7-anhydro-1,2-dideoxy-2-deuterio-D-gluco-oct-2- enitol, a prochiral substrate for probing the catalytic functioning of of glucosylases

Synthesis of the title compound provides a prochiral, glycosyl-donor substrate well suited for use as a probe of the catalytic functioning of D-glucosyl-mobilizing enzymes, because the full stereochemistry of enzymic reactions at its double bond may be unambiguously determined by examining the reaction products. The starting material for the synthesis was 2,6-anhydro-D-glycero-D-gulo-heptonic acid, from which 3,7-anhydro-4,5,6,8-tetra-O-benzyl-1-deoxy-D-glycero-D-gulo-2- octulose was prepared in eight steps. Reduction with lithium aluminum deuteride, and conversion of the resulting diastereomeric alcohols into (Z)-3,7-anhydro-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio-D- gluco-oct-2-enitol (11) and 3,7-anhydro-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio-D- glycero-D-gulo-oct-1-enitol (16), was carried out. By-products were 3,7-anhydro-2-O-benzoyl-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio -D-erythro-L-galacto-octitol and 3,7-anhydro-2-O-benzoyl-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio -D-erythro-L-talo-octitol, which could, like compound 16, be recycled. On debenzylation the oct-2-enitol 11 yielded (Z)-3,7-anhydro-1,2-dideoxy-2-deuterio-D-gluco-oct-2-enitol.     

On the regioselectivity of the Hanessian-Hullar reaction in 4,6-O-benzylidene protected galactopyranosides

The N-bromosuccinimide mediated fragmentation of methyl 4,6-O-benzylidene-beta-D-galactopyranoside results in the formation of methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside and methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside, as opposed to the methyl 6-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside originally reported. The kinetic methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside rearranges to the thermodynamic methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside under the reaction conditions, likely via a 3,6-anhydro galactopyranoside. The NBS-mediated cleavage of 4,6-O-benzylidene acetals in the galactopyranoside series is therefore shown to conform to the regiochemistry observed in the corresponding gluco- and mannopyranoside series with preferential cleavage of the C6-O6 bond by an ionic mechanism.     

Synthesis of (R)-2,3-epoxypropyl(1-->3)-beta-D-pentaglucoside

The title pentasaccharide was synthesized via a 2+3 strategy. The disaccharide donor, 3-O-acetyl-2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl-(1-->3)-2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranosyl trichloroacetimidate (8), was obtained by selective coupling of allyl 2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranoside with 3-O-acetyl-2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranosyl trichloroacetimidate (4), followed by deallylation, and trichloroacetimidation. Meanwhile, the trisaccharide acceptor, allyl 2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl-(1-->3)-2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl-(1-->3)-2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranoside (12), was prepared by coupling of allyl 2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranosyl-(1-->3)-2-O-benzoyl-4,6-O-benzylidene-beta-D-glucopyranoside with 4, followed by deacetylation. Condensation of 8 with 12, followed by epoxidation, and deprotection, gave the target pentaoside.     

Synthesis and anti-HIV activity of beta-D-3'-azido-2',3'-unsaturated nucleosides and beta-D-3'-azido-3'-deoxyribofuranosylnucleosides

Since the discovery of 3'-azido-3'-deoxythymidine (AZT) and 2',3'-didehydro-2',3'-dideoxythymidine (d4T) as potent and selective inhibitors of the replication of human immunodeficiency virus (HIV), there has been a growing interest for the synthesis of 2',3'-didehydro-2',3'dideoxynucleosides with electron withdrawing groups on the sugar moiety. Here we described an efficient method for the synthesis of such nucleoside analogs bearing structural features of both AZT and d4T The key intermediate, 3-azido-1,2-bis-O-acetyl-5-O-benzoyl-3-deoxy-D-ribofuranose, 5 was synthesized from commercially available D-xylose in five steps, from which a series of pyrimidine and purine nucleosides were synthesized in high yields. The resultant protected nucleosides were converted to target nucleosides using appropriate chemical modifications. The final nucleosides were evaluated as potential anti-HIV agents.     

(+/-)-1,2:5,6-Di-O-isopropylidene-myo-inositol and (+/-)-6-O-benzoyl-1,2:4,5-di-O-isopropylidene-myo-inositol: a practical preparation of key intermediates for myo-inositol phosphates.

A simple and practical synthetic procedure for the versatile intermediates, (+/-)-1,2:5,6-di-O-isopropylidene-myo-inositol and (+/-)-6-O-benzoyl-1,2:4,5-di-O-isopropylidene-myo-inositol, is described.     

Synthesis of alpha-galactosylated fragments related to the core-structure of the GPI anchor of Trypanosoma brucei

A series of octyl glycosides di- to tetrasaccharides related to the GPI anchor of Trypanosoma brucei was prepared. Treatment of octyl 2-O-benzoyl-4,6-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3 -diyl)-alpha-D-mannopyranoside with ethyl 2,3,4,6-tetra-O-benzyl-1-thio-beta-D-galactopyranoside under activation with bromine and silver trifluoromethanesulfonate afforded the alpha-linked disaccharide octyl 2-O-benzoyl-3-O-(2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-4,6-O- (1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-alpha -D-mannospyranoside, the siloxane ring of which was regioselectively opened with a HF-pyridine complex to give the disaccharide acceptor octyl 3-O-(2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-2-O-benzoyl-4-O-(3 -fluoro-1,1,3,3-tetraisopropyl-1,3-disiloxane-3-yl)-alpha-D- mannopyranoside (4). Mannosylation of 4 with benzobromomannose (7), followed by fluoride catalyzed desilylation gave the trisaccharide octyl 2-O-benzoyl-6-O-(2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl)-3-O-(2, 3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-alpha-D-mannospyranosi de, which was deblocked via the deacylated intermediate octyl 3-O-(2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-6-O-(alpha-D-manno pyranosyl)-alpha-D-mannospyranoside to afford the octyl glycoside trisaccharide octyl 3-O-(alpha-D-galactopyranosyl)-6-O-(alpha-D-mannopyranosyl)-alpha-D-m annospyranoside. Glycosylation of 4 with 3,4,6-tri-O-acetyl-2-O-(2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl)- alpha-D-mannopyranosyl trichloroacetimidate resulted in the tetrasaccharide octyl 2-O-benzoyl-4-O-(1-fluoro-1,1,3,3-tetraisopropyl-1,3-disiloxane -3-yl)-3-O-(2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-6-O-[2-O -(2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl)-3,4,6-tri-O-acetyl-alp ha-D-mannopyranosyl]-alpha-D-mannospyranoside, sequential desilylation, deacylation and debenzylation, respectively, of which via the intermediate octyl 2-O-benzoyl-3-O-(2,3,4,6-tetra-O-benzyl-alpha-D-galactopyranosyl)-6-O-[2 -O-(2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl)-3,4,6-tri-O-acetyl-a lpha-D-mannopyranosyl]-alpha-D-mannospyranoside afforded the octyl glycoside tetrasaccharide octyl 3-O-(alpha-D-galactopyranosyl)-6-O-[2-O-(alpha-D-mannopyranosyl)-alpha-D -mannopyranosyl]-alpha-D-mannospyranoside.     

Synthesis of capsular polysaccharide of Streptococcus pneumoniae type 14. 3. Synthesis of a monomer for polycondensation

Synthesis of a tritylated tetrasaccharide 1,2-O-(1-cyano) ethylidene derivative is described by glycosylation of 3,6-di-O-benzoyl-4-O-(2,4,6-tri-O-benzoyl-beta- D-galactopyranosyl)-1,2-O-[1-(exo-cyano)ethylidene]-alpha-D- glucopyranose with 6-O-acetyl-3-O-benzoyl-4-O-(2,3,4,6-tetra-O-benzoyl-beta- D-galactopyranosyl)-2-deozy-2-phthalimido-D-glucopyranosyl. bromide followed by selective deacetylation and tritylation.     

Stereoselective synthesis of 1,2:4,5-di-O-isopropylidene-3-C-(5-phenyl-1,2,4-oxadiazol-3-yl)-beta-D-psicopyranose and its X-ray crystallographic analysis

In a novel procedure, when 3-O-benzoyl-3-C-(N-hydroxycarbamimidoyl)-1,2:4,5-di-O-isopropylidene-beta-D-psicopyranose (1) is treated with acetic anhydride, chloroacetyl chloride, propanic anhydride and benzoyl chloride, the 3-O-benzoyl group undergoes an intramolecular replacement reaction with neighbouring group participation and transfer resulting in a more stable conjugated system by the formation of a 1,2,4-oxadiazol ring. A possible mechanism is reported. The structure has been determined by spectroscopic data and X-ray crystallographic analysis.     

Methyl 5-O-benzoyl-2,3-oxazole-D-ribofuranoside: a useful intermediate for the synthesis of conformationally restrained nucleosides

The synthesis of methyl 5-O-benzoyl-2,3-oxazole-D-ribofuranoside, a tetrahydrofuro [3,4-d]oxazole is described. The key step involves the reaction of methyl 3-amino-3-deoxy-5-O-benzoyl-D-ribofuranoside with N,N-dimethylformamide dimethyl acetal with cyclisation to the 2,3-oxazole via a prototropic rearrangement-elimination reaction.     

Synthesis of 6-aminohexylglycosides of a polysaccharide from Streptococcus group A and its fragments

Polycondensation of 4-O-benzoyl-1,2-O-(1-cyanoethylidene)-3-O-(3,4-di-O-benzoyl-2-O-tr ityl-alpha-L- rhamnopyranosyl)-beta-L-rhamnopyranose in the presence of 6-phthalimidohexyl-3,4-di-O-benzoyl-2-O-trityl-alpha-L- rhamnopyranoside affords, after deprotection, the polysaccharide built up of the repeating dissaccharide units----2) Rha (alpha 1----3) Rha (alpha 1----and containing 6-aminohexyl residue at the reducing end. This polysaccharide possesses the structure of the group A-variant streptococcal polysaccharide. Synthesis of 6-aminohexyl glycosides of 2- and 3-O-alpha-L-rhamnopyranosyl-alpha-L-rhamnopyranoses, which corresponds to the repeating units of the above polysaccharide, is described.